Applying nitride semiconductors to high-withstand-voltage and high-power semiconductor devices by utilizing their characteristics such as a high saturation electron velocity and a wide band gap has been considered. For example, GaN being a nitride semiconductor has a band gap of 3.4 eV, which is wider than a band gap of Si (1.1 eV) and a band gap of GaAs (1.4 eV), and has high breakdown electric field intensity. This makes GaN very promising as a material of semiconductor devices for power supply realizing a high voltage operation and a high power.
Many reports have been made on field-effect transistors, in particular, HEMTs (High Electron Mobility Transistors) as semiconductor devices using nitride semiconductors. For example, among GaN-based HEMTs (GaN-HEMTs), an AlGaN/GaN HEMT using GaN as an electron transit layer and using AlGaN as an electron supply layer has been drawing attention. In the AlGaN/GaN HEMT, a distortion ascribable to a difference in lattice constant between GaN and AlGaN occurs in AlGaN. Owing to piezoelectric polarization caused by the distortion and spontaneous polarization of AlGaN, high-concentration two-dimensional electron gas (2DEG) is obtained. Therefore, the AlGaN/GaN HEMT is expected as a high-efficiency switch element or a high-withstand-voltage power device for electric vehicles and the like.                [Patent Document 1] Japanese Laid-Open Patent Publication No. 2011-210750        
In recent years, in order to enable a higher-current operation in a semiconductor device using a nitride semiconductor, there has been actively studied a technique to implant ions to a nitride semiconductor layer under an ohmic electrode such as a drain electrode to reduce a contact resistance of the nitride semiconductor layer with the ohmic electrode. There has also been studied a technique to improve an operating current by increasing an Al composition ratio of an AlGaN layer of a nitride semiconductor.
However, in both of the techniques, the concentration of current density on an electrode end of the ohmic electrode is unavoidable. This concentration of the current density involves a concern that the ohmic electrode may suffer breakdown due to the current concentration on the electrode end at the time of a high-current operation which is desired in future.